I. Field of the Invention
Our invention relates to foldable fin erecting apparatus in general and, more specifically, to dynamic fin control systems for controlled erection of folding fins during flight.
II. Description of the Related Art
A variety of rockets, missiles, and other similar vehicles are known in the art. Many of these vehicles are designed for launch directly from storage containers or from confined storage volumes, either underwater, on the ground or airborne. Because such vehicles require fins for stabilization and control purposes during flight, the fins must be folded or retracted to a storage position so that a minimal storage volume is required. These retracted or folded fins must be moved from the storage position to a deployed position following vehicle launch.
Early practitioners installed a variety of springs and hydraulic actuators adjacent to the fin for fin deployment. Because controlled rotation in deploying the fin is desired, conventional deployment mechanisms tend to be mechanically complex and large, producing undesired aerodynamic drag during flight. Also, such large fin erection mechanisms increase the radar cross-section of the fin and thus increase the likelihood of undesired detection of the air vehicle.
Practitioners in the art have proposed methods for minimizing the size and complexity of these fin erection mechanisms by using uncontrolled erecting devices such as a spring-loaded hinge. A fundamental problem with such uncontrolled erecting devices is the excess energy that accumulates in the fin as it accelerates from the storage position to the deployed position. This rotational energy must be absorbed by some shock absorber means or by allowing the structure of the vehicle housing to deflect or deform as the fin hits the erect position stops.
Designing such an erection system to perform with acceptable deformations is made more difficult if the vehicle is not operated into the wind with a zero angle-of-attack. As the vehicle is launched, perturbations occur that result in a non-zero angle-of-attack for the air vehicle. For a typical air vehicle having a plurality of fins, the local fluid flow field at any individual fin may be widely varying. For instance, the windward fins experience a fluid flow that tends to hold the fins down (hindering wind) while the leeward fins experience a flow force that tends to push them into deployed positions (aiding wind). The windward fins may not erect if the hindering force is sufficient to overcome the uncontrolled erecting device and the leeward fins may move into deployed position with sufficient energy to damage the air vehicle housing upon impact with the deployment stops.
Existing folding fin technology evolved from early discoveries in aircraft wingtip control surface devices. U.S. Pat. No. 2,418,301, issued to L. C. Heal, discloses an aircraft supporting surface suitable for pivotable connection to the main wing or tail plane of an airplane. Heal discloses a hinged surface driven by a hydraulically-actuated mechanism that permits the aircraft to move a portion of the wingtips into vertical position and to control this vertical portion independent of the remainder of the wings. U.S. Pat. 2,565,990, issued to G. Richard, discloses a wingtip control surface suitable for permanent attachment as a vertical component at the tips of an aircraft wing. Richard's wingtip control surfaces are also independently controlled by hydraulic means.
U.S. Pat. No. 3,063,375, issued to Wilber W. Hawley, et al., discloses a folding fin erection scheme that permits the folding fin to be rotated in two dimensions during the erection process. Hawley, et al., teach the use of rocket booster thrust forces on the order of fifteen gravities (15g) as an aiding force for fin erection. Their invention is not suitable for use in air vehicles not having high launch accelerations.
U.S. Pat. No. 4,323,208, issued to James Ball, discloses a folding fin assembly for a flight vehicle in which a gearing arrangement controls the relationship between fin rotations in two dimensions from storage to deployment. Ball relies on aerodynamic and inertial thrust forces to force the fin into a deployed position, and his gearing transmission operates to passively hold a fixed relationship between erection angle and fin control angle.
While Ball suggests that active motor means could be used to force the fin into position, he does not consider the problems of overcoming hindering wind forces or controlling aiding wind forces to prevent damage to air vehicle housing caused by excessive fin deployment momentum nor does he suggest a workable control scheme for active fin deployment.
U.S. Pat. No. 4,334,657, issued to Kjell Mattson, discloses a fin-stabilized projectile assembly wherein a plurality of fins are mounted on the tail section. Each fin is spring-loaded in a manner that pushes it into a deployed position immediately following launch of the projectile. Mattson teaches a completely passive erection means and does not consider the problem of housing damage because of the robust projectile housing suitable for use with his invention.
U.S. Pat. No. 4,457,479, issued to Martine Doude, discloses a winglet apparatus for aircraft wingtips having an active control system for automatically moving the winglets between an aerodynamically optimal angle-of-attack and a minimal wing bending moment angle-of-attack in response to stresses acting on the wing. Doude teaches the use of automatic moving means for optimizing the winglet effect as a function of the flight parameters and wing stress, thereby avoiding the need for structural reinforcement of the wings to accommodate the additional bending moments acting on the wings because of the presence of the winglets. However, he does not consider the application of his control schemes to the fin deployment problems known in the art.
U.S. Pat. No. 4,624,424, issued to George T. Pinson, discloses a missile yaw and drag controller actuator system having a plurality of control surfaces operated by an actuator drive. The actuator drive positions the surfaces to catch the fluid flow along the missile housing but cannot effect steering control at low missile velocities. U.S. Pat. No. 4,699,333, also issued to George T. Pinson, discloses a similar actuator-controlled panel system for missile roll control.
U.S. Pat. No. 4,714,216, issued to Spencer D. Meston, et al., discloses a fin erecting mechanism wherein the fin is rotatable about a pivot from an initial storage position to a deployed position and the erection is essentially spring-powered. Meston, et al., teach the use of a single spring for uncontrolled deployment and latching in the deployed position but do not suggest solutions to the above problems known in the art.
U.S. Pat. No. 4,884,766, issued to Harold F. Steinmetz, discloses an automatic fin deployment mechanism housed within the air flight vehicle that employs a pyrotechnic gas generator to drive the fin from storage to deployment. Steinmetz, et al., teach the Use of a clutch means that can be disengaged from the fin to permit fin rotation in a second dimension, but their invention is essentially an uncontrolled fin erection mechanism.
Other investigators such as Messerschmitt (German Patent No. DE3508-103-A) disclose fin erection mechanisms powered by the aerodynamic forces generated in the fluid flow over the vehicle housing. However, these investigators suggest no means for controlling the energy build-up in the unfolding fin to prevent housing damage on impact at the deployed position. Neither do they consider the problem of aerodynamic force variation from fin to fin on air flight vehicle bodies having multiple fins.
All these problems must be resolved for a fin design that is steerable and controllable when it is in its deployed position without interfering with proper fin control during flight and without investing in large, expensive and troublesome fin erection mechanisms. These unresolved problems and deficiencies are clearly felt in the art and are solved by our invention in the manner described below.